It is an object of the invention to provide a hydraulic unit which is capable of supplying defined variable volume flows without the risk of air aspiration.
The hydraulic unit comprises a multiple piston pump composed of individual pumps arranged in a specific array, wherein the reservoir has connected therewith an auxiliary reservoir at least partly filled with oil for filling up the reservoir when oil has been withdrawn by the multiple piston pump, thus ensuring a complete submersion of the multiple piston pump.
A multiple piston pump is a positive displacement pump supplying a continuous volume flow which can be varied by changing the rotational speed. If the motor is a synchronous motor, the magnitude of the volume flow is adapted to be varied in accordance with the load of the respective power consumer using a corresponding motor control. Use of a multiple piston pump, where the individual pumps are arranged in a specific array in the reservoir, involves the risk that, when the oil level decreases, the intake opening of at least one individual pump is temporarily not submerged and takes in air. This must be prevented in view of obtaining a defined volume flow.
According to the invention, the auxiliary reservoir makes sure that the reservoir is filled up again when a large quantity of oil has been withdrawn from the reservoir, such that a complete submersion of the multiple piston pump is ensured.
The hydraulic unit according to the invention is in particular suitable in cases where consumers with different consumption capabilities are connected. If a consumer is a hydraulic power wrench for turning screws and bolts, the load to be overcome by the hydraulic pressure is normally high such that the delivery rate and/or the volume flow are small. A different scenario is provided if a connected consumer is a piston-cylinder unit, for example, whose large-volume cylinder includes a piston which is moved against a small load. Here, a resultant rapid piston movement produces a large volume flow. The large volume flow results in a change of the oil level in the reservoir, wherein the upper individual pumps are possibly not submerged for a short time. Such a condition is prevented by the auxiliary reservoir.
The invention is advantageously applicable to a hydraulic unit comprising a reservoir with a cross section where the width of the reservoir narrows towards the upper end. Such a cross section is offered e.g. by a reservoir having a substantially round cross section. Such a reservoir allows a space-saving placement of the motor and the pump. This configuration offers a hydraulic unit with a small volume and a low weight such that the hydraulic unit can be designed as a portable unit. On the other hand, without an auxiliary reservoir there would be the risk that due to the cross section narrowing in upward direction, the upper region having a small volume would rapidly be emptied and remain so when a large quantity of oil is withdrawn.
The auxiliary reservoir may be arranged on top of the reservoir either as an additional container or as a cavity which is permanently connected with the reservoir, but has a larger width than the upper end of the reservoir.
According to a preferred aspect of the invention, the auxiliary reservoir is hermetically sealed above the oil level, and a connecting line connects the auxiliary reservoir with a vented additional tank at a location below the oil level. Here, the auxiliary reservoir and the additional tank define a communicating system. The atmospheric pressure causes oil to be fed from the vented tank to the auxiliary reservoir when oil is withdrawn from the auxiliary reservoir to flow to the reservoir. The additional tank increases the volume of the auxiliary reservoir, but, on the other hand, is spatially separated from the auxiliary reservoir. The auxiliary reservoir is arranged above the reservoir, while the additional tank may be located below the reservoir. The additional tank may thus define a base for the housing of the reservoir.
Further, a plurality of additional tanks of different sizes may be provided which are optionally adapted to be attached to the housing. In this manner, the user can select the size of the respective additional tank.
Alternatively, the auxiliary reservoir can be used without any additional tank. In this case, a ventilation opening must be provided above the maximum oil level such that the withdrawal of oil from the reservoir is not affected.